[9] RFR (S): 8059340: ConstantPool::_resolved_references is missing in heap dump

[Coleen Phillimore]

Vladimir,

I think that changing the format of the heap dump isn't a good idea either.

On 4/16/15, 2:12 PM, Vladimir Ivanov wrote:
> (sorry for really late response; just got enough time to return to the 
> bug)

I'd forgotten about it!
>
> Coleen, Staffan,
>
> Thanks a lot for the feedback!
>
> After thinking about the fix more, I don't think that using reserved 
> oop slot in CLASS DUMP for recording _resolved_references is the best 
> thing to do. IMO the change causes too much work for the users (heap 
> dump analysis tools).
>
> It needs specification update and then heap dump analyzers should be 
> updated as well.
>
> I have 2 alternative approaches (hacky and not-so-hacky :-)):
>
>   - artificial class static field in the dump ("<resolved_references>" 
> + optional id to guarantee unique name);
>
>   - add j.l.Class::_resolved_references field;
>     Not sure how much overhead (mostly reads from bytecode) the move 
> from ConstantPool to j.l.Class adds, so I propose just to duplicate it 
> for now.

I really like this second approach, so much so that I had a prototype 
for moving resolved_references directly to the j.l.Class object about a 
year ago.  I couldn't find any benefit other than consolidating oops so 
the GC would have less work to do.  If the resolved_references are moved 
to j.l.C instance, they can not be jobjects and the 
ClassLoaderData::_handles area wouldn't have to contain them (but there 
are other things that could go there so don't delete the _handles field 
yet).

The change I had was relatively simple.  The only annoying part was that 
getting to the resolved references has to be in macroAssembler and do:

go through method->cpCache->constants->instanceKlass->java_mirror() 
rather than
method->cpCache->constants->resolved_references->jmethod indirection

I think it only affects the interpreter so the extra indirection 
wouldn't affect performance, so don't duplicate it!  You don't want to 
increase space used by j.l.C without taking it out somewhere else!

>
> What do you think about that?

Is this bug worth doing this?  I don't know but I'd really like it.

Coleen

>
> Best regards,
> Vladimir Ivanov
>
> On 10/6/14 11:35 AM, Staffan Larsen wrote:
>> This looks like a good approach. However, there are a couple of more 
>> places that need to be updated.
>>
>> The hprof binary format is described in 
>> jdk/src/jdk.hprof.agent/share/native/libhprof/manual.html and needs 
>> to be updated. It’s also more formally specified in hprof_b_spec.h in 
>> the same directory.
>>
>> The hprof JVMTI agent in jdk/src/jdk.hprof.agent code would also need 
>> to be updated to show this field. Since this is a JVMTI agent it 
>> needs to be possible to find the resolved_refrences array via the 
>> JVMTI heap walking API. Perhaps that already works? - I haven’t looked.
>>
>> Finally, the Serviceability Agent implements yet another hprof binary 
>> dumper in 
>> hotspot/agent//src/share/classes/sun/jvm/hotspot/utilities/HeapHprofBinWriter.java 
>> which also needs to write this reference.
>>
>> Thanks,
>> /Staffan
>>
>> On 29 sep 2014, at 16:51, Vladimir Ivanov 
>> <vladimir.x.ivanov at oracle.com> wrote:
>>
>>> http://cr.openjdk.java.net/~vlivanov/8059340/webrev.00/
>>> https://bugs.openjdk.java.net/browse/JDK-8059340
>>>
>>> VM heap dump doesn't contain ConstantPool::_resolved_references for 
>>> classes which have resolved references.
>>>
>>> ConstantPool::_resolved_references points to an Object[] holding 
>>> resolved constant pool entries (patches for VM anonymous classes, 
>>> linked CallSite & MethodType for invokedynamic instructions).
>>>
>>> I've decided to use reserved slot in HPROF class header format.
>>> It requires an update in jhat to correctly display new info.
>>>
>>> The other approach I tried was to dump the reference as a fake 
>>> static field [1], but storing VM internal 
>>> ConstantPool::_resolved_references among user defined fields looks 
>>> confusing.
>>>
>>> Testing: manual (verified that corresponding arrays are properly 
>>> linked in Nashorn heap dump).
>>>
>>> Thanks!
>>>
>>> Best regards,
>>> Vladimir Ivanov
>>>
>>> [1] http://cr.openjdk.java.net/~vlivanov/8059340/static
>>